Gas Chromatography-Mass Spectrometry (GC-MS)

Gas chromatography-mass spectrometry technology, referred to as GC-MS. It is a chromatographic-mass spectrometry technology that combines gas chromatograph (GC) and mass spectrometer (MS) through appropriate interface components, supplemented by corresponding data collection and control systems. This technology uses gas chromatography as a means for sample separation and preparation, and mass spectrometry as an on-line detection means for the separated components of gas chromatography for qualitative and quantitative analysis. Gas chromatography and mass spectrometry control, data collection and analysis are all completed by powerful computer technology. GC-MS has become a mature conventional analysis technology widely used in chemical engineering, environment, agriculture, biomedicine, etc.

Technical Principle

Gas chromatography is a physical separation method. Using the difference in the distribution coefficient (solubility) of each component of the measured substance between the mobile phase (gas phase) and the stationary phase at a certain temperature, the different components are repeatedly distributed between the two phases in relative motion to achieve separation. Gas chromatography has high-efficiency separation ability and good sensitivity, and can perform effective qualitative and quantitative analysis of complex mixtures. The basis of qualitative analysis of gas chromatography is retention time, and the basis of quantitative analysis is chromatographic peak height and peak area. However, there are obvious limitations in the way of using the retention time of the component to characterize it. Therefore, gas chromatography may not be able to accurately identify the components, and the qualitative ability is poor.

Mass spectrometry is an analytical method for measuring the charge-to-mass ratio of ions. The basic principle is to convert the vaporized sample molecules into charged ions in the ion source, and then accelerate the formed ions into the mass analyzer. In the mass analyzer, the fragment ions are collected and recorded in the order of the charge-to-mass ratio to obtain a mass spectrum. Perform qualitative analysis of samples through the corresponding library search. Quantitative analysis of samples can be performed based on the relative intensity of the mass spectrum peaks. Mass spectrometry has strong structure identification capabilities, high sensitivity, and strong qualitative specificity. However, the limitation of mass spectrometry is that it cannot identify complex mixtures, and the sample needs to be a single component.

Gas chromatography can efficiently separate mixtures, but its qualitative ability is poor and cannot accurately identify individual components. The mass spectrometer has strong qualitative ability and can accurately identify single components, but it is powerless to analyze the mixture. Therefore, people combined the two methods to form a gas chromatography-mass spectrometer. In GC-MS, gas chromatography is the sample preconditioner of mass spectrometry, which is responsible for separating the mixture to obtain a single component, while mass spectrometry is the detector of gas chromatography, which is responsible for the analysis of single components. The combined use of the two gives full play to their respective expertise, so that the separation, identification and quantification of samples can be carried out at the same time.

Among all the hyphenated technologies, the GC-MS technology is the earliest, the most developed, the most widely used, and the least difficult. Since the sample after separation from the gas chromatography column is in a gaseous state, the mobile phase is also a gas. This is in line with the sampling requirements of the mass spectrometer, so the two instruments are the easiest to use together. Moreover, the GC-MS technology combines the advantages of gas chromatography and mass spectrometry, and makes up for their respective defects, so it has the characteristics of strong sensitivity, fast analysis speed, and strong identification ability. GC-MS can complete the separation and identification of components to be tested at the same time, and is especially suitable for qualitative and quantitative analysis of complex mixtures, structure identification of compounds, molecular weight determination of compounds and elemental composition analysis.

Instrument structure

The GC-MS instrument is mainly composed of four parts: gas chromatography system, mass spectrometer, interface and computer system.

GC-MS instrument structure diagram

• Gas Chromatography System

The gas chromatography system consists of injector, chromatographic column and microprocessor that controls the chromatographic conditions. The gas chromatography system effectively separates the components in the sample and meets the requirements of the sample unity of mass spectrometry. Gas chromatography requires the sample to have a low boiling point and good stability. Gas chromatographic columns currently mostly use small-bore capillary chromatographic columns.

• Interface

The interface sends the components flowing out of the gas chromatograph to the mass spectrometer for detection, and acts as an adapter between the gas chromatograph and the mass spectrometer. The interface has two functions in GC-MS. One is to reduce the pressure of the carrier gas, so that the pressure at the outlet of the positive pressure gas chromatography column and the pressure at the negative pressure mass spectrometer ion source match each other. The second is to eliminate excess carrier gas, so that the measured component is concentrated and enters the ion source.

• Mass spectrometry system

The mass spectrometer system consists of an ion source, a mass analyzer, and a microprocessor of the detector. The mass spectrometry system analyzes the components sequentially introduced into the interface and becomes the detector of the gas chromatography system. From the perspective of the ion source of the mass spectrometry system, the mass spectrometry system of GC-MS mainly includes electron ionization (EI), positive chemical ionization (PCI) and negative chemical ionization (NCI). Since EI requires that the sample to be tested must be vaporized, it is most appropriate to use EI for GC-MS.

• Computer system

The computer system interactively controls the gas chromatograph, interface and mass spectrometer, performs data acquisition and processing, and is the control unit of the GC-MS.

Application

GC-MS is playing an increasingly important role in the food industry, environmental monitoring, petroleum industry, life sciences, biomedicine and other fields.

In the field of environmental monitoring, GC-MS can be used to analyze pollutants in the atmosphere, such as toxic and harmful gases, gas sulfides, and nitrogen oxides in the atmosphere. GC-MS can also analyze polycyclic aromatic hydrocarbons, organic solvents and pesticide residues in drinking water, and analyze organic pollutants in water resources and soil.

In the food industry, GC-MS can be used for component analysis and quality evaluation of volatiles in food additives, flavors and fragrances, and food materials.

In the petroleum industry, GC-MS can be used for the analysis of crude oil, refinery gas, gasoline additives, hydrocarbon compounds, nitrogen-containing compounds, and sulfur-containing compounds.

In the field of biomedicine, GC-MS is a standard method to detect and characterize volatile drugs and poisons under GC conditions. For small molecular metabolites such as amino acids, fatty acids, polysaccharides, vitamins, cholesterol, polyols, polyamines, hormones, peptides, and nucleotides in biological samples such as urine, blood, and saliva, GC-MS is an analytical technique with the advantages of high sensitivity, strong resolution, good reproducibility, and high throughput.